Sizing of ethylene polymer structures



Patented Apr. 20, 1954 SIZING F ETHYLENE POLYMER STRUCTURES Thomas. Franklin Banigan, Kenmore, N. Y., as- Signor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application Septemberlli, 1951, Serial No. 246,527

Claims.

This invention relates to improvements in films, sheets, filaments, fibers, monofils and lik structures of solid ethylene polymer, and more particularly to improvements in the surface of ethylene polymer structures of high transparency.

Films of ethylene polymer (i. e., polyethylene) are highly useful in a great variety of packaging applications such as for fresh produce, frozen foods, dried milk, chemicals and pharmaceuticals, textiles, etc. A description of films made from ethylene polymer is embodied in U. S. P. 2,219,700, issued to Perrin et al. These films have excellent inherent moistureproofness, good mechanica1 strength and are highly resistant to the action of many chemicals; and, though they are described as being translucent to transparent, all ethylene polymer films heretofore commercially available have been translucent. Filaments, staple fibers, monofils and the like are described in U. S. P. 2,210,774.

Films, filaments, fibers, monofils, coatings, etc. of solid ethylene polymer, however, have a propensity to develop stron electrostatic charges which make them difiicult to manipulate during manufacture and subsequent processing, such as spinning, knitting, Weaving, slitting, printing, sealing, etc. By reason of their highly charged surfaces, these articles also accumulate heavy dust deposits which, of course, are very undesirable.

Further, ethylene polymer films are smooth and very flexible, and superimposed sheets of the material separate with difficulty, particularly Whenpressure, even though moderate, is applied. In continuous roll form, the films do not. slide easily over automatic packaging machinery, resulting in very unsatisfactory operation.

Heretoiore, a number of sizing compositions have been developed for improving the surface properties of solid ethylene polymer structures, 0. i., U. S. P. 2,519,013 to T. F. Banigan, wherein is disclosed a sizing composition comprisin a water-soluble alkyl aryl polyglycol ether. These sizing compositions are highly efficient as antistatic and slip promoting agents for solid ethylene polymer structures such as the translucent film heretofore available. However, techniques have recently been developed for producing commer cially a transparent solid ethylene polymer film; and it has beenfound that the sizing compositions which have previously been satisfactorily used for sizin translucent film are unsuitable for transparent film, for the reason that in every instance the-sizing composition does not improve both slip and anti-static properties.

An object of'thls invention, therefore, is to improve the surface characteristics of films, filaments, fibers, monofils, sheets, coatings, etc. of solid ethylene polymer whereby to enhance the slip thereof and to inhibit the accumulation of electrostatic charges thereon, without detracting from the transparency, flexibility, strength, moistureprooiness, chemical resistance, heat sealability, etc. of the ethylene'polymer structures.

Another object is to provide novel compositions for improving the surface characteristics of solid ethylene polymer structures, especially films, sheets and coatings. These and other objects will more clearly appear hereinafter.

The above objects are accomplished according to the present invention by treating the surfaces of a solid ethylene polymer structure with an aqueous composition consisting of water, a dispersed polyvinyl acetate, an alkali metal alkyl sulfate wherein the alkyl group contains at least eight carbon atomszinc'chloride, and, optionally, sodium chloride, removing the excess composition and drying the treated structures.

The polyvinyl acetate particles deposited upon the film surface provide for good slip and excellent heat scals. It has been found that the type and size of the polymer particle are relatively critical in providing for-good slip. For example, it has been found that the polymer particles should be as nearly spherical in shape as possible. Furthermore, the particle must have an approximate particle sizeof at least 3 microns, but should not'exceed 12 microns. Preferably, the amount of polyvinyl acetate in'the compositlon should be in-the'range of 0.10'to 2.0%,based on the weight of water. Concentrations below the lower limits'are ineffectual for'producing best results. On the'other hand, concentrations of the dispersed polymer substantially greater than 2% result in the'formation of a hazy coating which is highly undesirable in coating substantially transparent solid ethylene polymer film.

As examples of the alkali metal alkyl sulfates in which the alkyl group contains 8 or more carbon atoms, there may be mentioned: sodium oleyl sulfate, sodium lauryl sulfate, sodium stearyl sulfate, sodium myristyl sulfate, sodium cetyl sulfate, etc, or mixtu'es thereof. The preferred concentration of alkali metal alkyl sulfate is from 0.15 to 0.50%, based .on the weight of water. Concentrations substantially greater than 0.50% produce-a 'somewhat 'hazysurface in most cases.

Excessive concentrations ofizinc chloride tend to reduce the strengthofhe'at seals. Further, excessive concentrations of zinc chloride should definitely be avoided because of toxicity, especially when the resulting film is to be used for wrapping food. In general, not more than 0.3% of zinc chloride, based on the weight of water, should be used. As little as 0.05% zinc chloride may be employed with good result.

Sodium chloride functions chiefly as an antiblocking agent; and its presence, although not required, is preferred for this reason. Crystals of the salt precipitate upon the film surface, and these crystals function to separate adjacent film layers and, hence, prevent film-to-film sticking. Sodium chloride crystals should be used in a concentration within the range of 0.15 to 0.50%, based on the weight of the water.

Application of the aqueous dispersion to the surface of the ethylene polymer structure may be accomplished by any convenient expedient such as by dipping, spraying, brushing, roll coating, etc., preferably at a temperature of from (SO-70 C. Excess aqueous dispersion may be removed by squeeze rolls, doctor knives, etc. The dispersion should be applied in such amount that there will be deposited from 6 to 45 milligrams, and, preferably, from 15 to 30 milligrams, of solids per square yard of surface of the ethylene polymer structure.

- The following examples will serve to further illustrate the principles and practice of my invention. Parts and percentages are by weight unless otherwise indicated.

Example I Melt-extruded ethylene polymer film 0.002 of an inch thick, having a haze value of 26 (transparent) was passed through an aqueous dispersion maintained at about 65 C. containing 0.25 particulate polyvinyl acetate having a particle size of from 3-12 microns, 0.15% sodium lauryl sulfate, 0.15% of sodium chloride, and 0.10% zinc chloride, all based on the weight of the water disperse medium. The dilute dispersion was prepared by adding small portions of more concentrated aqueous polyvinyl acetate (55-60% solids) to the proper amount of water. After passing through the aqueous dispersion, the film was passed between rubber squeeze rolls to remove the excess dispersion, and dried. The resulting sized film was of high clarity, and the haze value thereof did not difier substantially from that of the unsized film. In addition, the electrostatic propensity of the treated film at 75 F. and 35% R. H. (relative humidity) was 91 volts, as compared with 560 volts for unsized film; and the coefficient of friction (film-to-film) at 75 F. and 75% R. H. was 0.52 (initial) and 0.33 (running), as compared with 235+, both initial and running, for unsized film.

The electrostatic propensity characteristic of each film hereinafter set forth for purposes of comparison, was measured as follows:

A strip of film, 1 inch by 4 inches in size, is supported on and in electrical contact with a fiat, slightly roughened stainless steel plate which is connected to a meter for measuring electrical potential, but which is otherwise insulated from its surroundings. The plate is roughened by rubbing it lightly with #100/ 120 Carborundum cloth, making very slight scratches at right angles to the direction of movement of the film. A weighted, felt-covered block which has a 1 inch by 2 inch face, presses the film into contact with the stainless steel plate. An electrostatic charge is generated by pulling the film for a distance of 3 inches between the weighted block and the stainless steel plate; and the charge so generated on the film discharges on to the stainless steel plate with which it is in contact, and is measured by the meter. The values registered on the meter (herein called electrostatic propensity) are purely relative and are useful only for purposes of comparison. They indicate the ability of a film to accumulate an electrostatic charge under the influence of friction normally encountered in manipulating or working with the film.

The coefiicient of friction of each film was determined as follows:

A rectangular plate of metal linked to an indicator is placed on top of a sheet of polyethylene film supported on a smooth, fiat surface. The sheet is then pulled under the metal piece and over the supporting surface at a steady rate. As the metal plate adheres to the moving film, it pulls on the indicator, the extent of the pull being in proportion to the friction between the plate and film. The coefficient of friction is somewhat higher when the film is first pulled under the block (initial) than when it is running steadily (running). The metal block will give a film-tometal coefiicient of friction and when the block is Wrapped in several layers of film and then tested, a film-to-film coefficient of friction is obtained.

Example II Following the procedure of Example I, film of the same specifications was sized with an aqueous dispersion containing 0.25% polyvinyl acetate, 0.15% sodium lauryl sulfate, 0.10% sodium chloride, and 0.05% zinc chloride. The sized film was of high clarity; had an electrostatic propensity of 140 volts at 75 F. and 35% R. ii; and the coefiicient of friction (film-to-film) at 75 F. and 75% R. H. was 0.68 (initial) and 0.38 (running), as compared with 236+, both initial and running, for unsized film.

Example III Transparent ethylene polymer fi 0.002 of an inch thick, was sized as in the preceding examples with an aqueous dispersion containing 0.75% polyvinyl acetate, 0.20% sodium lauryl sulfate, 0.15% sodium chloride, and 0.05% zinc chloride. The resulting clear, transparent film exhibited an electrostatic propensity of 140 volts at 75 F. and 35% R. H. The coeificient of friction at 75 F. and 75% R. H. was 0.50 (initial) and 0.30 (running).

In all of the foregoing examples, the heatsealing properties of the resultant film were excellent; and at heat-sealing temperatures ranging from 130 C., the seal strength was about 1500+ grams (see Ubben, U. S. P. 2,147,130).

It is to be understood that the sizing compositions of the present invention are not only satisfactory for substantially transparent solid ethylene polymer film, but also for translucent films and solid ethylene polymer in other forms. Furthermore, the sizing compositions of the present invention are an improvement over heretofore known sizing compositions insofar as antistatic and slip-promoting properties are concerned.

As many widely differing changes may be made without departing from the spirit and scope of my invention, it is to be further understood that said invention is in no wise restricted save as set forth in the appended claims.

.I claim:

1. Structures of ethylene polymer the surfaces of which have uniformly distributed thereon from 6 to 45 milligrams per square yard of surface of a composition comprising essentially particulate polyvinyl acetate having a particle size of from 3 to 12 microns, an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, and zinc chloride.

2. Structures of ethylene polymer the surfaces of which have uniformly distributed thereon from 6 to 45 milligrams per square yard of surface of a composition consisting of particulate polyvinyl acetate having a particle size of from 3 to 12 microns, an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, sodium chloride, and zinc chloride.

3. Films of ethylene polymer the surfaces of which have uniformly distributed thereon from 6 to 45 milligrams per square yard of surface of a composition consisting of particulate polyvinyl acetate having a particle size of from 3 to 12 microns, an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, sodium chloride, and zinc chloride.

4. Transparent films of ethylene polymer the surfaces of which have uniformly distributed thereon from 6 to 45 milligrams per square yard of surface of a composition consisting of particulate polyvinyl acetate having a particle size of from 3 to 12 microns, an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, sodium chloride, and zinc chloride.

5. A method for improving the anti-static and slip characteristics of structures of ethylene polymer which comprises coating the surfaces of said structures with a dilute aqueous dispersion containing from 0.10% to 2% by weight, based on the weight of water, of dispersed particulate polyvinyl acetate having a particle size of from 3-12 microns, from 0.15% to 0.50% of an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, and from 0.05% to 0.3% of zinc chloride, at a temperature of from 60-70 C1, and thereafter drying said coated structures.

6. A method for improving the anti-static and slip characteristics of structures of ethylene polymer which comprises coating the surfaces of said structures with a dilute aqueous dispersion containing from 0.10% to 2% by weight, based on the weight of water, of dispersed particulate polyvinyl acetate having a particle size of from 3-12 microns, from 0.15% to 0.50% of an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, from 0.15% to 0.50% of sodium chloride, and from 0.05% to 0.3% of zinc chloride, at a temperature of from (SO- 0., and thereafter drying said coated structures.

7. The method of claim 6 wherein the structure is film.

8. The method of claim 6 wherein the structure is transparent film.

9. A sizing composition for enhancing antistatic characteristics and for promoting slip characteristics of structures of ethylene polymers, comprising essentially water, from 0.10% to 2% by weight, based on the weight of the water, of particulate polyvinyl acetate having a particle size of from 3-12 microns dispersed in the water, from 0.15% to 0.50% of an alkali metal allryl sulfate containing at least 8 carbon atoms in the alkyl group, and from 0.05% to 0.3% of zinc chloride.

10. A sizing composition for enhancing antistatic characteristics and for promoting slip characteristics of structures of ethylene polymers consisting of from 0.10% to 2% by weight, based on the weight of the water, of particulate polyvinyl acetate having a particle size of from 3-12 microns dispersed in the water, from 0.15% to 0.50% of an alkali metal alkyl sulfate containing at least 8 carbon atoms in the alkyl group, from 0.15% to 0.50% of sodium chloride, and from 0.05% to 0.3% of zinc chloride.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,005,619 Graves June 18, 1935 2,176,053 Billing Oct. 17, 1939 2,279,501 Dickey et a1. Apr. 14, 1942 2,279,502 Dickey et al. Apr. 14, 1942 2,358,355 Stamatoff Sept. 19, 1944 2,413,428 Billings Dec. 3, 1946 

5. A METHOD FOR IMPROVING THE ANTI-STATIC AND SLIP CHARACTERISTICS OF STRUCTURES OF ETHYLENE POLYMER WHICH COMPRISES COATING THE SURFACES OF SAID STRUCTURES WITH A DILUTE AQUEOUS DISPERSION CONTAINING FROM 0.10% TO 2% BY WEIGHT, BASED ON THE WEIGHT OF WATER, OF DISPERSED PARTICULATE POLYVINYL ACETATE HAVING A PARTICLE SIZE OF FROM 3-12 MICRONS, FROM 0.15% TO 0.50% OF AN ALKALI METAL ALKYL SULFATE CONTAINING AT LEAST 8 CARBON ATOMS IN THE ALKYL GROUP, AND FROM 0.05% TO 0.3% OF ZINC CHLORIDE, AT A TEMPERATURE OF FROM 60-70* C., AND THEREAFTER DRYING SAID COATED STRUCTURES. 